1. Field of the Invention
The present invention relates to a voltage/current driven active matrix organic electroluminescent (EL) pixel circuit capable of performing voltage and current programming using one pixel circuit and an organic EL display device using the same.
2. Discussion of Related Art
In general, a method of driving a flat panel display is divided into a passive driving method and an active driving method. According to the active driving method, a thin film transistor (TFT) that operates as a switch and a storage capacitor that stores data are included in each pixel. Such an active driving method is divided into a voltage driving method and a current driving method. According to the voltage driving method, the final output of data programmed in a pixel circuit is in the form of voltage. According to the current driving method, the final output of data programmed in the pixel circuit is in the form of current. Such voltage and current driving methods vary in accordance with a liquid crystal device mounted in the flat panel display. An organic EL display device is a display device driven by current.
FIG. 1 is a circuit diagram of a conventional voltage driven active matrix organic EL pixel circuit. As shown in FIG. 1, the conventional voltage driven active matrix organic EL pixel circuit includes two TFTs T1 and T2, a storage capacitor Cs, and an organic light emitting diode (hereinafter, referred to as OLED). In FIG. 1, T1 is a switching TFT which turns on and off in response to a gate signal transmitted to a gate line and transmits data in a source line to one end of the storage capacitor Cs such that the data is programmed in the storage capacitor Cs. T2 is a voltage driven TFT and operates as a voltage source for driving a power source applied to a power line in accordance with the data programmed in the storage capacitor Cs to emit light from the OLED to a predetermined level.
FIG. 2 is a circuit diagram of a conventional current driven active matrix organic EL pixel circuit. As shown in FIG. 2, the conventional current driven active matrix organic EL pixel circuit includes four TFTs T1, T2, T3, and T4, a storage capacitor Cs, and an OLED. In FIG. 2, T1 and T2 are switching TFTs which turn on and off in response to a gate signal transmitted to a gate line and transmit data in a source line to one end (N1) of the storage capacitor Cs such that the data is programmed in the storage capacitor Cs. T3 and T4 are current driven TFTs and operate as a current mirror type source for driving a power source applied to a power line in accordance with the data programmed in the storage capacitor Cs to emit light from the OLED to a predetermined level.
On the other hand, the conventional voltage driven active matrix organic EL pixel circuit illustrated in FIG. 1 has a simple structure and is similar to a conventional voltage driven LCD such that a driving IC can be used as it is. However, picture quality significantly deteriorates due to non-uniformity in TFTs and it is very difficult to control grayscales since the organic EL is a current driven device.
Also, according to the conventional current driven active matrix organic EL pixel circuit illustrated in FIG. 2, it is possible to directly control current such that it is possible to compensate for deterioration in picture quality due to non-uniformity in TFTs and to easily control the gray scales. However, the conventional current driven active matrix organic EL pixel circuit has a complicated structure and requires a driving IC only for the organic EL.